1. Field of the Invention
The present invention relates to a distributed feedback (DFB) semiconductor laser which can change its polarization mode of output light according to its pumped condition. The semiconductor laser can be used as a signal light source for optical transmission and the like. The invention also relates to a method of using the distributed feedback semiconductor laser.
2. Description of the Related Art
A conventional oscillation polarization mode selective DFB semiconductor laser as disclosed in Japanese Patent Application Laid-Open No. 7(1995)-162088, for example, uses a multi-electrode structure to control a relative relation between the wavelength dependency of gains created in the laser's active layer and Bragg wavelengths determined by a pitch and the like of its diffraction grating.
The following device has been developed and proposed as an oscillation polarization mode selective dynamic single mode semiconductor laser. The oscillation polarization mode selective device has a structure that can be modulated by a digital signal produced by superposing a minute-amplitude digital signal on a bias injection current. The device is a distributed feedback (DFB) laser in which a distributed reflector of a grating is introduced into a semiconductor laser resonator or cavity and wavelength selectivity of the grating is utilized. In the device, strain is introduced into an active layer of a quantum well structure, or its Bragg wavelength is located at a position shorter than a peak wavelength of a gain spectrum, so that gains for the transverse electric (TE) mode and the transverse magnetic (TM) mode are approximately equal for light at wavelengths close to an oscillation wavelength, under a current injection condition near an oscillation threshold. Further, a plurality of electrodes are arranged and currents are unevenly injected through those electrodes. An equivalent refractive index of the cavity is unevenly distributed by the uneven current injection, and oscillation occurs in one of the TE mode and the TM mode and at a wavelength which satisfies a phase matching condition and takes a minimum threshold gain. When the balance of the uneven current injection is changed slightly to vary a competitive relation of the phase condition between the TE mode and the TM mode, the oscillation polarization mode and wavelength of the device can be switched.
In that semiconductor device, an antireflection coating is provided on one end facet to asymmetrically employ effects of the uneven current injection into its output-side portion and its modulation-electrode portion. Alternatively, electrode lengths are varied to introduce a structural asymmetry.
FIG. 1 illustrates such a conventional structure. FIG. 1 is a cross-sectional view taken along a light propagation direction of the DFB semiconductor laser. In its structure, a lower clad layer 1010, an active layer 1011, a light guide layer 1012, an upper clad layer 1013, and a cap layer 1014 are laid down on a substrate 1009. A diffraction grating g is formed at an interface between the light guide layer 1012 and the upper clad layer 1013. The cap layer 1014 is divided into two along a cavity direction by a separating groove 1015. Currents can be injected into two electrically-independent active layer regions (portions under electrodes 1002 and 1003) by using the electrodes 1002 and 1003 formed on the cap layer 1014 and an electrode 1008 formed on a bottom surface of the substrate 1009. An antireflection film 1004 is provided on one end facet of the device.
Further, Japanese Patent Application Laid-Open No. 2(1990)-159781 discloses a three-electrode DFB semiconductor laser with a .lambda./4 phase shift section which can be used as a semiconductor laser for switching its lasing polarization mode between the TE mode and the TM mode. In the semiconductor laser, currents can be independently injected into its central region with the .lambda./4 phase shift section and two regions formed on both sides of the central region. Here, the lasing polarization mode can be switched between the TE mode and the TM mode by changing the current injected into the central region under a uniformly-injected current condition.
Furthermore, Japanese Patent Application Laid-Open No. 2(1990)-117190 discloses a semiconductor laser apparatus which has two semiconductor devices arranged serially or in parallel that primarily generate or amplify light waves in a predetermined polarization mode and another polarization mode, respectively.
However, the two conventional oscillation polarization mode selective DFB semiconductor lasers above select the TE or TM mode depending on the phase condition, and are sensitive to the phase at the end facet. As a result, the lasing wavelength and polarization mode of the device depend on the current injection condition in a complicated fashion, and fluctuation in characteristics of the lasing polarization mode appears among individual devices. In particular, regarding the fluctuation among devices, it is difficult to selectively impart a gain to one of the competing polarization modes when a current injected into one of two waveguide portions is increased, so that a lasing polarization switching point varies among the devices.
Further, in those two conventional lasers, the phase relations of resonant light in the cavity for respective polarization modes are controlled by the balance of currents injected into a uniform structure extending in the cavity direction (a uniform active layer and a uniform diffraction grating), and the lasing polarization mode is thus switched by changing the polarization mode whose oscillation threshold gain is the smallest. Therefore, those two lasers suffer the following disadvantages.
(1) Since the light phase is controlled, the lasing polarization mode is again returned to the TE mode, for example, when the switching from the TE mode to the TM mode is conducted by increasing a current injected into a certain region, and thereafter this current is further increased.
(2) Due to the multi-electrode structure, a common polarization mode is not selected among plural regions when currents injected into those regions increase. The respective regions are thus brought into independent lasing conditions, and those conditions influence each other to cause a plurality of longitudinal modes.
On the other hand, in the above third conventional DFB semiconductor laser, the light wave in the predetermined polarization mode is generated or amplified based on its geometrical shape, so that its yield varies due to processing fluctuations of etching depth and edge width during its ridge fabrication process.